Typical conductive pastes have been prepared by kneading a conductive powder in the presence of binding resins (binders) and solvents. Films such as electric resistive films and electroconductive films have been manufactured from such conductive pastes. The conductive powder and binder resins are not specifically limited in the variations and contents, but they are appropriately determined depending on the target values such as resistance or the like of the above-mentioned films. Typically, the conductive powder is selected from the group consisting of conductive carbon black and powders of metals such as gold, silver, copper, nickel, palladium and aluminum, or powders of alloys of these metals. The binding resins are selected from thermosetting resins such as phenolformaldehyde resin, xylene modified phenol resin, epoxy resin, melamine resin, and acrylic resin.
Recently however, electric resistive films etc. are required to have a higher heat-resistance to be used under high temperature conditions and to correspond with increasing heat output caused by the trend for smaller devices with higher capacitance. To solve this problem, conductive pastes using polyimide precursor resins as binding resins are disclosed (Unexamined Published Japanese Patent Application (Tokkai-Sho) 52-12459, Tokkai-Sho 54-78492, and Tokkai-Sho 54-136694). General polyimide precursors used for conductive pastes also are disclosed in Tokkai-Hei 3-233904, Tokkai-Hei 7-22214. Disclosures about improvement of properties of binding resins include an epoxy modified polyimide precursor (Tokkai-Sho 60-176006) and a siloxane modified polyimide precursor (Tokkai-Sho 58-7473).
However, in the above-mentioned polyimide precursors used for conductive pastes, the weight average degree of polymerization (n) is at least 70, that is, the polyimide precursors generally have a high molecular weight. Moreover, they have linear molecular structures and a high intermolecular cohesion. As a result, the polyimide precursors are less soluble in solvents, that is, they may be dissolved only in special solvents with high water absorptivity, and thus, only limited solvents can be used.
Since the above-mentioned polyimide precursors have less solvent solubility, a conductive paste manufactured by using such a polyimide precursor also will have a low solid content concentration inevitably. When such a conductive paste is used for screen printing, printing should be carried out several times since thick printing film cannot be obtained in a single printing step, so that printing and temporary drying should be repeated alternately up to forming a desired film thickness. This has generated serious problems in the manufacturing process, for example, it complicates the printing process, takes time and raises costs.